The invention relates to a process of making superfine amorphous silicic acid by reacting alkali-metal silicate solutions with an acid or an acidic substance and, if desired, in the presence of neutral salts. These silicic acids are distinguished by a very fine grain size, high structure and very good dispersing properties in water and organic media.
Specific types of silicic acids are known which have a structure different from that of normal silicic acids. These are wet-precipitated silicic acids or silica gels which form high "structure" products by virtue of specific modifications of the drying process. These products include aerogels, silica gels or silicic acids in which the intermicellar liquid prior to drying consists of organic solvents or mixtures of such solvents with water. In this group belong also spray-dried silicic acids and silica gels.
Aerogels have previously been obtained by drying of so-called organogels under critical conditions. This process is very expensive because of the necessity to use large amounts of relatively expensive organic solvents and the employment of autoclaves which are discontinuously operated at extreme drying temperatures (high pressures and high temperatures). The aerogels have therefore as a rule been used only for special purposes. It is also known to replace the intermicellar liquid (water) in normal hydrogels or silicic acids partly or entirely by corresponding organic solvents. Through the stage of organogels or organosilicic acids there are thus likewise obtained SiO.sub.2 -containing materials with an increased structure at normal drying conditions. This process, similar to the one for making aerogels, also requires the use of more expensive organic solvents.
More recently, the use of spray-dried silicic acid has become accepted for certain applications. These types of silicic acids are likewise distinguished by a higher structure, though this structure is not comparable with that of the types previously discussed. The spray-drying, apart from the requirement of rather expensive appliances, is not possible without an increased use of energy input. The products made by this process therefore can likewise be used only for special purposes.
From German Pat. No. 1,000,793 a process is known whereby alkali-metal silicate solutions are reacted with acids or acidic-acting substances, if desired in the presence of neutral salts. This process results in well-dispersible finely divided silicic acid if the precipitation of the silicic acid is effected by a rapid and intensive mixing of the reaction component by application of a high shearing force based on a steep speed gradient. This high speed gradient in these cases is produced by a colloid mill.
In this patent, it has also been proposed to carry out the precipitation step in two tanks or vessels of different size. The alkali-metal silicate solution is initially disposed in the larger tank and is continuously pumped throughout the entire precipitation step up to its completion through the smaller vessel. The result is the mixing of the alkali-metal silicate solution with the acid in the smaller precipitation vessel which latter must have a volume relative to the total volume of the reaction solution amounting to a specific value, that is, between 1:2 and 1:100 and preferably between 1:4 and 1:20. The difference in the alkali-metal content of the portion of the reaction mass which is discharged from the precipitation tank and the fraction which at the same time enters the precipitation tank should always be less than 10% and preferably less than 3% relative to the alkali-metal contents of the initial alkali-metal silicate solution.
The silicate acids made by this process are distinguished by a higher structure compared with conventionally made precipitated silicic acids and are particularly useful as reinforcing fillers in synthetic rubbers. However, this process is likewise rather costly and cumbersome because of the use of precipitation tanks of different sizes and because of the long periods necessary for the dispersion and the thus required high energy input.
The discussion so far makes it quite apparent that the making of high structure silicic acids either requires special drying methods or expensive precipitation apparatus. The thus-obtained products can therefore be used only for special applications.
Since all these silicic acid types have a structure different from the products obtained in conventional precipitation processes, it is desirable to determine the presence and size of structure by accurate measurements. In the following the concept of the so-called "structure" and the methods for determining it by accurate measurements shall therefore be discussed.
"Structure of a silic acid" is meant to refer to the size of the cavities or three-dimensional measurements which arise from the loose aggregation of primary particles to form more-or-less stable secondary particles or aggregates. These cavities, though in form of capillaries, do not have measurements as strictly defined as the cavities in, for instance, a porous body. The size of the so-called "intergranular volume" of the secondary particles of silicic acid permits, however, to draw conclusions regarding the packing density of the primary particles, and thus regarding the "structure" of the silicic acid.
In order to determine the capillary volume of a silicic acid, it is possible to use the absorption of a more-or-less defined absorption liquid as physical measuring standard.
In case of the structure of silicic acids, it is possible to arrive at a measurement of the oil absorption by means of the so-called Brabender-Cabot method (a) V. A. Sljaka et al, Cabot-release for ASTM D-24-Meeting 1-5-63 and (b) E. R. Eaton, J. S. Middleton (Cabot Corp.) Rubber World, 152, 94-100 (1965).
FIG. 1, which will be identified further below, illustrates the plastogram of a high structure silicic acid. The consistency of the oil-silicic acid mixture, measured in units of angular momentum, is recorded as a function of the time (in minutes) or as a function of the added amount of oil. From the diagram of FIG. 1 it is thus possible to readily determine the viscosity maximum and the amount of oil which was added up to the point where specific landmark points were reached on the viscosity curve. This amount of oil is stated as specific oil number in ml oil per gram.
The measurements were carried out with the Brabender-Cabot absorptometer which was developed by the Cabot Company of the United States in collaboration with the W. C. Brabender Instrument Company. This apparatus is particularly equipped for oil absorption investigations in carbon blacks and fillers and is provided with a special Cabot kneader. The Cabot kneader is connected with a Brabender plastograph. Its kneading chamber is open at the top and is covered by a plate which prevents the entry of dust into the silicic acid powder in the initial phase. After adding oil in a continuous flow to the kneader there is reached a point where the oil-silic acid mixture starts to agglomerate whereupon the angular momentum rises first slowly and then very steeply. This agglomeration is followed by cake formation (see FIG. 1). The cakes are fragments of the total mass which are in a pulpy consistency and are escaping the mixing action because of the open kneading chamber and thus subsequently start dancing on the kneader paddles. Oil that is then added does wet these cakes at their surface; it can, however, no longer be admixed homogeneously.
This process becomes apparent in the viscosity curve of the oil-silicic acid mixture (see FIG. 1), which shows that the curve outline, after initial slow rising, advances steeply towards a maximum and subsequently, while strongly oscillating around a mean value of the angular momentum (effect of the cake formation), will then decrease again upon further addition of oil. If the curve structure up to the agglomeration and the steep rise of the angular momentum is well reproducible for comparative tests, this is not true regarding the maximum value which depends on the cake formation and therefore relative to the added amount of oil is subject to slight changes (see J. Behre, G. Schramm in Gummi, Asbest, Kunststoffe 19, 912-922 (1966).
The proposed ASTM norm avoids the possible measurement errors by adding oil--not up to the maximum but--up to a preceding more-or-less arbitrary limit value of the angular momentum, for instance a value of 70% of the maximum. This limit value which must be ascertained by tests for each type of silicic acid is therefore a condition of the reproducible determination of the oil absorption by filling materials.
The oil absorption value according to the Cabot method is determined as the amount of oil in ml/g filler which must be added up to reaching the said limit value. Preferably, dibutyl-phthalate (DBP) is used as the absorption liquid. The oil absorption is therefore expressed as the DBP number.
This method permits to accurately determine the structure of a precipitated silicic acid and to make an appraisal of the possible uses of silicic acids made by different processes as depending on the structure of the acid.
Since the special applications, particularly in the areas of the pesticide and lacquer technologies, the requirements as to quality of the employed silicic acid are very high, it is one of the objects of the present invention to make silicic acids with a well-defined and reproducible high structure while avoiding the shortcomings of the previously described prior-art processes.
A still more specific object of the invention is a finely distributed high structure silicic acid in the range of from 1 to 15 microns having a high DBP or oil number which acid can be obtained by suitable precipitation conditions and optimum mechanical action on the SiO.sub.2 suspension and a controlled modification of the secondary particle distribution.
It is thus an object also of the invention to reduce the relatively high energy requirement of the process disclosed in German Pat. No. 1,000,793, that is to limit the time of the dispersing action to a minimum and thus to improve the economics of the process.